1. Field of the Invention
The present invention relates to a method of producing reduced iron and production facilities therefor, and particularly relates to production facilities for annexing a rotary bed-type direct reducing furnace with a sealed-type electro-blast furnace, a pellet production facility including the rotary bed-type direct reducing furnace and a few improved methods of producing pellets containing reducing materials, and a production facility and method for supplying the pellets into a rotary bed-type direct reducing furnace, and further relates to compositions of raw materials and binders for producing pellets.
2. Background Art
Conventionally, reducing iron is obtained by the steps of forming pellets using mixed raw materials of iron ore and a carbonaceous material by means of pellet production facilities, and by reducing the pellets by means of a direct reducing furnace.
The reduced iron pellets are obtained by forming raw material pellets which are a mixture of raw materials such as an oxide particulate of iron ore and a particulate of carbonaceous material like coal and by heating these pellets at high temperature in a rotary bed-type direct reducing furnace. The thus produced reduced iron pellets are then placed into a melting furnace for meting the reduced pellets and for producing pig iron. Conventionally, a sealed type blast furnace, a so-called submerged arc-furnace is used as the melting furnace, and will be described later.
Production of reduced iron by the rotary bed-type furnace is advantageous in many respects such as (1) it is possible to utilize coal with a reduced cost as the reducing agent, (2) it is possible to acquire a favorable heat economy, since preheating of the raw material pellets prepared by raw materials is not necessary, (3) the reduction reaction can be completed within a short time (approximately 10 min.), because the materials to be reacted (the iron ore powder and the carbonaceous material powder) are in close contact with each other. Therefore, it is possible to construct the rotary bed-type direct reducing furnace in a simple structure and at a reduced cost.
Hereinafter, a conventional rotary bed-type direct reducing furnace will be explained in detail with reference to FIGS. 15 and 16.
FIG. 15 is a perspective view of the conventional rotary bed-type direct reducing furnace (hereinafter, referred to as a reducing furnace). The reference numeral 101 is the reducing furnace, 122 is a burner provided through the side wall of the reducing furnace, 113 is a transporting device for charging the pellets into the reducing furnace, 102 is a conveyer such as a reciprocating conveyer for charging the pellets in a uniformly layer on the rotary bed, 124 is a pellet discharging device such as a screw-type discharging device for discharging reduced pellets (reduced iron pellets) from the reducing furnace, 104 is a container for temporary storing the reduced iron pellets, and 105 is an exhaust gas duct for discharging the combustion gas in the reducing furnace.
The raw material pellets produced by mixing the iron ore powder as the oxide material and a coal powder as the carbonaceous material is supplied to the reciprocating conveyer 102 through the transporting device 113. The raw material pellets are charged into the reducing furnace 101 uniformly as one or two layers on the rotary bed 127. This pellet filled layer is heated by the burner 122 at about 1200° C. and reduced during turning one round in the direction shown by an arrow (clockwise revolution in FIG. 15) in the reducing furnace 101, and the reduced pellets are discharged by the screw type discharging device 124 from the reducing furnace 101, and stored in the container 4. The combustion gas in the reducing furnace 101, after heating the raw material pellets, is discharged through the exhaust dust 105 to the outside the reducing furnace 101, and released into the atmosphere.
FIG. 16 is a cross-sectional developed diagram along the center of the reducing furnace clockwise showing the pellet charging portion and the reduced pellet discharging portion. In FIG. 16, the reference numeral 103 denotes a reduced iron pellets discharging chamber (discharging portion) for discharging the reduced iron pellet layer 128. The rotary bed 127 of the reducing furnace 101 is made of a refractory, its bottom is constructed by a steel member, and wheels are annexed under the steel member. The reference numeral 131 is a rail for these wheels. The rotary bed 127 of the rotary bed-type reducing furnace 101 rotates such that it moves from left to right in a cross-sectional and developed diagram along the center axis of the reducing furnace The burner 122 is used for heating the raw material pellets and air for combustion is supplied to the burner through an air line 123. The reference numeral 134 is a combustion flame, and 125 is a cooling device for cooling the pellet layer indirectly in order to prevent the reduced iron pellet layer from burning. The reference numeral 126 is a damper for sealing the gas flow between the reduced iron pellet discharge chamber and the raw material pellet charging chamber 102.
As hereinabove described, a conventional facility for producing the reduced iron pellets having high mechanical strength is described in detail. Although the reduced iron pellets are produced by the above-described rotary bed-type direct reducing furnace, the other systems may be used for producing reduced iron which comprises a few annexed equipment.
FIG. 17 illustrates a facility containing a perspective view of the rotary bed-type direct reducing furnace 201, wherein the raw material pellets are supplied onto the rotary bed 206 by a charging conveyer 202 through a charging conveyer 204. The inside chamber of the rotary bed-type direct reducing furnace 210 is heated by use of a plurality of burners (not shown).
As shown in FIG. 17, a reduced iron pellet production facility comprises a conventional rotary bed-type direct reducing furnace 201, shown as a cross sectional developed diagram, for producing the reduced iron pellets by heating the raw material pellets P1 at high temperatures, and a rotary cylinder-type cooler 202 for cooling the reduced iron pellets P2 preserved at high temperature after receiving them at a transfer portion 208 during rolling.
The reference numeral 203 denotes a pelletizer for producing pellets of the mixture of the raw materials comprising an iron ore powder and a coal powder. The reference numeral 204 denotes a transporting device such as a belt conveyer, 205 denotes a pellet charging device for charging pellets on the bed 206 into a uniform pellet layer 207.
The reference numeral 208 denotes a discharging device for discharging the heated reduced iron pellets P2 into the rotary cylinder-type cooler 202, and 209 denotes an exhaust gas duct for discharging the combustion gas.
The reference numeral 210 shown in FIG. 4 denotes a cylindrical chute for delivering the reduced iron pellets P2 heated at a temperature of 1100° C. into the rotary cylinder-type cooler 202 and 211 denotes a gas sealing hood.
The reference numeral 212 denotes spray nozzles for spraying cooling water on the outside surface of the rotary cylinder-type cooler 202, and 213 denotes a spray nozzle for spraying cooling water directly on the reduced iron pellets located near the exit port of the cooling cylinder 102. The reference numerals 214 and 215 denote rubber-rollers for supporting the rotating rotary-cylinder-type cooler 202, and 216 denotes a gear for driving the rotary cylinder-type cooler. The numeral 217 denotes a hood as well as a hopper, 218 denotes a sieving screen, and 219 denotes an exhaust duct of water vapor.
Conventionally, although the same rotary base-type reducing furnace as shown in FIG. 15 is used, another method for producing reduced iron pellets is known, in which, wet raw material pellets formed by adding water, mixing, and pelletizing are heated and reduced, without prior drying, in the rotary bed-type reducing furnace shown in FIG. 18 which is the same as that shown in FIG. 15.
A facility for producing the reduced iron pellets by use of wet raw material pellets is shown in FIG. 19, including the rotary bed-type reducing furnace 301.
Referring to FIG. 19, the reference numeral 306 denotes a storage tank for the iron oxide powder such as the iron ore powder, 307 a storage tank for the carbonaceous powder such as a coal powder, and 308 a storage tank for a binder material such as bentonite. A kneading machine 309 kneads materials weighed and collected from these storage tanks 206, 207, and 208, while adding water, and a pelletizing machine 310 produces wet pellets by adding water to the kneaded powders as the material for production of reduced iron pellets. On the other hand, the reference numeral 301 denotes the rotary base-type reducing furnace which is the same as that shown in FIG. 16. The wet raw material pellets are charged continuously on the rotary bed 320 by a pellet charging machine 302 such as a conveyer through a transporting device 311 so as to form a uniform wet pellet layer 312.
As the rotary bed rotates (the rotational direction is shown by an arrow A), wet raw material pellets 312 on the rotary bed 320 are heated and reduced. The numeral 303 denotes a discharging portion of the reduced iron pellets at high temperature, 304 denotes a container for temporary storing the heated pellets, and 305 a exhaust gas duct for discharging the combustion gas in the reducing furnace 201.
In order to improve the mechanical strength in terms of the falling distance of the raw material pellets without being fractured, the type and added amounts were studied. When the raw material pellets are formed as described above, generally, the iron ore powder which mainly contains Fe2O3 and Fe2O4 and the carbonaceous material powder such as coal or coke powders are mixed and pelletized into pellets by addition of a binder. As the binder for forming pellets, usually bentonite is incorporated in a range of 0.5 to 1 wt %.
The raw material pellets containing the reducing material, after being pelletized by the pelletizing machine and dried by a drying machine, is charged into the reduced iron producing machine, which is the same rotary base-type direct reducing furnace as that shown in FIG. 15 or 16. Accordingly, the raw material pellets undergo various mechanical impact during transportation or charging on the rotary bed.
FIG. 21 is a diagram for explaining the process using the rotary bed-type reducing furnace. The reference numeral 401 denote a reducing furnace, which has the same structure as shown in FIG. 15. The cross-sectional view of the reducing furnace 401 is shown in FIG. 21. The rotary bed 412 is a disc formed from a belt-like plate, and wheels 413 are provided beneath the rotary bed so as to engage with the rail 414 constructed concentrically about the center axis of the rotary bed. The rotary bed 412 is driven by a driving mechanism (not shown) and rotates on the rail at a certain rotating speed.
As shown in FIG. 21, fine powders of the iron ore and the coal are mixed with the binder, pelletized into pellets (10 to 12 mm in diameter) by a pelletizing machine 402, and dried by a drying machine 403. After being dried, the raw material mixed pellets are charged on the rotary bed of the reducing furnace 401 by a charging machine 404 which will be described later. The pellets moves together with the movement of the rotary bed.
As shown in FIG. 23, a plurality of burners 407 are provided along the outside periphery of the reducing furnace, and a high temperature combustion gas is generated by combustion of a fuel. The high temperature combustion gas circulates in the furnace in the direction opposite to that of the rotational direction (shown by an arrow A) of the furnace, and the inside of the reducing furnace is maintained at high temperature. This high temperature atmosphere reduces Fe2O3 and Fe3O4 in the iron ore and the combustion gas is then discharged to a preheater 409 of primary air for combustion and released to the air after passing through a dust collector 410.
The raw material pellets are directly reduced during one circulation in the rotary bed reducing furnace and the thus reduced pellets containing reduced iron are discharged from the reducing furnace by means of a screw-type discharging machine 405 (FIG. 24) located near the pellet charging portion and the reduced pellets are supplied to the subsequent process after being cooled by a cooler 406.
A pellet charging machine 404 is provided for charging the raw material pellets on the rotary bed as a layer of pellets having a uniform thickness (the thickness of one or two pellets layer).
Here, the raw material pellets charging machine for charging the raw material on the rotary bed of the rotary bed-type direct reducing furnace will be described.
FIGS. 24 and 25 show the pellet charging machine, which comprises a receiving portion 516 of the raw material pellets, an inclined plate 516b for guiding the pellets discharged from the receiving bin 516, and a pair of partition plates 517 and 518 for controlling the thickness of the pellet laminate layer. The height of the opening H3 at the bottom of the receiving portion 516, the intervals H2 and H1 under the partition plates are set as H3>H2>H1. The pellets in the receiving bin 516 is supplied onto the rotary bed 512 after passing through intervals of H3, H2, and H1 which becomes sequentially smaller.
A first problem arises in the conventional rotary bed-type direct reducing furnace 1 as shown, for example, in FIGS. 15 and 16 in that the inner pressure of the raw material pellet charging portion 150 is made negative by an exhaust fan for discharging the combustion gas, because the pellet charging portion 150 is close to the exhaust duct 105 and is opened to the outside atmosphere. In addition, a space is formed in between the pellet charging portion 150 and the charging conveyer 102, and it is likely to cause air flows in the directions 153 and 154.
Even though a damper 126 is formed to prevent air from flowing between the pellet charging portion 102 and the reduced iron pellet discharging portion 103, the leak air is flown into the pellet discharging portion as shown by an arrow 155 due to insufficient seal. Thereby, the first problem encountered is that the reduced iron pellets at high temperature are again oxidized according to the following chemical reactions caused by the leak air flown into the reducing furnace 101.Fe (reduced iron)+½O2→FeO  (1) Fe (reduced iron)+¾O2→½Fe2O3  (2) 
Sometimes, the combustion gas is made to flow into the pellet discharging portion 103, because the internal pressure balance in the pellet charging portion 150 and the pellet discharging portion 103 is destroyed due to the air flow 55. Since the combustion gas is an oxidizing gas containing CO2 gas and H2O gas, the reduced iron is re-oxidized by the following chemical reactions.Fe+CO2→FeO+CO  (3) Fe+H2O→FeO+H2  (4) 
The second problem concerns the mechanical strength of the reduced iron pellets discharged from the reducing furnace. That is, the reduced iron pellets produced in the rotary bed-type direct reducing furnace have a very low density and a very low crushing strength, corresponding to very low mechanical strength. Practically, the crushing strength of one reduced iron pellet with a diameter of 10 mm is as low as approximately 30 kgf, which is too fragile to be shattered when such pellets are thrown in a blast furnace as the raw material.
The reason for the weak strength is because the gas phase including oxygen, carbon, and a volatile component (when coal is used as the carbonaceous material) is released from the raw material pellets and also because the reduction time in the reducing furnace 1 is too short to sinter the pellets after the gas release.
The third problem concerns the wet raw material pellets. If the wet raw material pellets are charged onto the rotary bed 227 heated more than 700° C. of the rotary bed-type reducing furnace 201, the wet raw material pellets will be fractured by the explosion of steam generated by the rapid heating of the wet raw material pellet. This fracture by the steam explosion is called bursting, and the bursting naturally reduces the yield of the reduced iron product.
In order to avoid such a bursting phenomenon, the surface temperature at the charging portion (Ti) of the rotary bed is reduced below 700° C. as shown in FIG. 20. As shown in FIG. 20, the surface temperature at the discharging portion (To) is as high as 1100° C. In order to establish the temperature gradient between the discharging portion (To) and the charging portion (Ti), it is necessary to have a considerable distance between the charging portion and the discharging portion, which requires the expanded rotary bed and the increased cost of equipment, which constitute the third problem. The point (b) in FIG. 9 represents the point where the wet pellets are dried and the temperature at the point (b) in the furnace is the lowest.
The fourth problem is related to the composition of the raw material pellets, because the conventional raw material pellets additionally containing bentonite are likely to be fractured by mechanical shocks applied to the raw material pellets during transportation. When dried, the downfall strength of the raw material pellets containing bentonite becomes so weak that almost all of the pellets are fractured when they are dropped from a height of 300 mm. If the raw material pellets are fractured or subjected to surface peeling, the diameters of the pellets becomes uneven which causes uneven reduction in the reducing furnace and which also causes uneven quality of the reduced iron. In order to eliminate the fracture of the raw material pellets, the conventional transportation equipment are designed so as to reduce the falling height of pellets when transporting or charging, which results in reducing the degree of freedom in designing the transportation and charging apparatuses. It is an object of the present invention to provide a method of obtaining the raw material pellets having a high downfall strength.
Furthermore, the fifth problem remains which is related to the charging apparatus of the raw material pellets. As shown schematically in FIG. 10, a problem of the conventional charging apparatus arises in that the raw material pellets are easily broken when the thickness of the pellet layer is controlled by means of the partition plates.
That is, since it is necessary for the charging apparatus to charge a comparatively large amount of pellets onto the rotary bed of the rotary bed-type reducing furnace, the raw material pellets are subjected to a large amount of tensile stress, when pellets are damed by the partition plates 317, 318 for reducing the thickness of the pellet layer, until the pellets are supplied onto the rotary base 327. Therefore, when the pellet charging machine is used, the raw material pellets are broken or the surfaces of the pellets are peeled off. The fracture, breakage, or the surface peeling make pellet diameters uneven, which results in uneven reduction of pellets in the rotary base reducing furnace and this in turn results in making it difficult to maintain the quality of the reduced iron pellets.
As shown in FIG. 7, when the thickness of the charged pellet layer is controlled by the partition plates 317, 318, since the rotary base is in a form of circular belt type, the outer peripheral speed of the rotary base differs from that of the inner peripheral speed, which results in changing the density of the resultant reduced iron pellets. That is, when the conventional pellet charging machine is used, the distribution along the radial direction changes, so that the raw material pellets cannot be uniformly distributed on the rotary base of the reducing furnace. Thereby, the quality of the reduced iron can not be preserved due to the uneven diameters of the pellets by the fracture or peeling of the pellets.